In an almost incomprehensible twist of fate, an Australian woman who lost her brother in the disappearance of Malaysia Airlines Flight 370 learned on Friday that her stepdaughter was on the plane shot down over Ukraine.

Ukraine accused pro-Russian separatists of shooting down a Malaysian jetliner with 298 people aboard, sharply escalating the crisis and threatening to draw both East and West deeper into the conflict. The rebels denied downing the aircraft.

American intelligence authorities believe a surface-to-air missile brought the plane down Thursday but were still working on who fired the missile and whether it came from the Russian or Ukrainian side of the border, a U.S. official said.

American intelligence authorities believe a surface-to-air missile took down a passenger jet in eastern Ukraine on Thursday, a U.S. official said, but the Obama administration was still scrambling to confirm who launched the strike and whether there were American citizens killed in the crash.

As unthinkable as shooting down an airliner with hundreds of passengers is, it has happened before. Among the most notable cases in recent decades were an Iranian plane shot down by the U.S. Navy and a South Korean airliner destroyed by a Russian fighter jet.